1. Field of the Invention
The present invention concerns a method for determining the properties of planar or foil-like materials of high moisture content and low electrical conductivity.
The invention also involves an apparatus for the implementation of the method.
2. Description of Background Art
The application of conventional measurement equipment in the determination of properties of sheet- and foil-like materials moving rapidly on a production line is often difficult. The transducers used are mainly of a contacting type, such as humidity sensors for plywood, based on resistance measurement, or sheet basis weight measuring scales. Typically, the contacting transducers, e.g., capacitive sensors, are susceptible to a high measurement error by the vertical movement of the material. Transducers based on penetrating radiation, such as gamma rays, often have a slow response, are expensive, and, furthermore, are hazardous due to the ionizing radiation utilized. In contrast, a nonpenetrating radiation, such as infrared radiation, delivers only superficial information from the immediate surface of the material.
With the use of radio frequency radiation, many of the aforementioned drawbacks and sources of error can be eliminated. The noncontact measurement of material becomes feasible, and the measurement also conveys information about the inside of the material sheet. Due to the inexpensive construction of the transducer construction itself, several transducers can be advantageously mounted in a parallel arrangement, which facilitates a fast measurement across the entire line. Furthermore, by measuring a plurality of factors related to the propagation of radio waves in the material, a concurrent determination of material characteristics is possible. However, even when using RF transducers, the movement of the sheet or foil relative to the transducer often causes large errors in measurement since, in the measurement of thin sheets or foils, the transducer must be brought close enough to the material to obtain a sufficiently strong signal.